In using conventional pressurised metered dose inhalers with some hydrofluoroalkane (HFA) formulations being developed by the applicant company it has been found that medicament tends to deposit in the valve stems of the canisters thereof. It will be appreciated that this deposition could lead to the user receiving an unusually high dose of medicament if the deposit were at least in part to break away or, indeed, result in the valve stems becoming blocked, thereby rendering the pressurised metered dose inhalers inoperative. Ideally, one would simply just wash the canisters to remove the deposit. Unfortunately, however, it is not possible to wash the canisters as the formulations are sensitive to moisture and would as a result of washing be contaminated.